1. Field of the Invention
The present invention relates to a semiconductor device in which a semiconductor chip is mounted, a method for fabrication thereof, an image sensor device for reading image information from a written/printed manuscript and the like, and a method for fabrication thereof.
2. Description of the Related Art
There are conventionally known methods for mounting a semiconductor chip such as an LSI (Large Scale Integrated Circuit) on a wiring board. Examples of such methods are a wire-bonding method, a flip-chip-bonding method, a film-carrier-bonding method, and a beam-lead-bonding method.
In a wire-bonding method, the semiconductor chip is fixed by means of an electrically conductive adhesive on a substrate having a circuit conductor layer formed thereon. Electrodes on the semiconductor chip are connected to the circuit conductor layer by means of metal wires such as those made of Au or Al. However, the bonding process in a wire-bonding method is troublesome using the metal wires. Moreover, it is necessary to form the electrodes of the semiconductor chip in a peripheral portion of the semiconductor chip, making it difficult to reduce the pitch between adjoining electrodes. Therefore, it is difficult to apply such a wire-bonding method to mounting of a semiconductor chip having a large number of electrodes. In order to solve such a problem, U.S. Pat. Nos. 5,065,006 and 5,138,145 each discloses a method for mounting an image sensor chip in an image sensor device, as a method for mounting a semiconductor chip through a wireless-bonding method.
The methods for producing a conventional image sensor device, as are disclosed in the above patents, will be described with reference to FIGS. 8 and 9. FIG. 8 is a cross-sectional view showing a conventional image sensor device, in which an image sensor chip 16 is mounted face-down on a transparent substrate 11.
The image sensor device shown in FIG. 8 is fabricated in the following manner: As is shown in FIG. 8, a photo-curable insulating resin 18 is applied on the transparent substrate 11. The transparent substrate 11 has a circuit conductor layer 12 formed thereon. Thereafter, the image sensor chip 16, which includes electrodes 15 and light-sensitive elements 19 such as photodiodes, is pressed face-down on the transparent substrate 11. While a pressure is applied, the photo-curable insulating resin 18 is irradiated with ultra violet rays through the transparent substrate 11, so that the photo-curable insulating resin is cured, and the image sensor chip 16 is fixed on the substrate. Finally, a resin such as that of a silicon type is applied thereon to form a passivation layer 17. Thus, the image sensor device is fabricated.
FIG. 9 shows a step in the fabrication process of another conventional image sensor device. The image sensor device shown in FIG. 9 is fabricated in the following manner: As is shown in FIG. 9, an image sensor chip 26 has electrodes 25 formed thereon. On each electrode 25, another electrode (bump) 24 is formed. Each bump 24 has a ring tab formed at a tip portion thereof. The bumps 24 abut a circuit conductor layer 22 formed on a substrate 21 when the image sensor chip 26 is mounted face-down on the substrate 21. By applying a pressure on the image sensor chip 26, the ring tabs of the bumps 24 collapse, so that the circuit conductor layer 22 and the bumps 24 are connected to each other in a close contact.
As is described above, in the first conventional method, the photo-curable insulating resin is cured by being irradiated with ultra violet rays through the transparent substrate. Therefore, a portion of the photo-curable insulating resin which is shaded by the circuit conductor layer formed on the transparent substrate may not be sufficiently irradiated with the ultra violet rays, and therefore may not be cured completely. In such cases, since the photocurable insulating resin is not completely cured, the physical connection between the semiconductor chip and the substrate is insufficient, causing the device to have poorer physical endurance. Moreover, the semiconductor chip and the circuit conductor layer are likely to have poor electrical connection. Furthermore, in cases where the device is an image sensor device, since the insulating resin is not completely cured, optical characteristics thereof are likely to lack uniformity, causing various problems. For example, if the resin has portions having different refractive indices, light incident to the device is refracted and/or scattered thereby. As a result, the light-sensitive element tends to read images with errors, or the resolution of the read images are degraded.
With respect to the second conventional method, since the ring tabs of the bumps are collapsed by applying a certain level of pressure or more thereto, the image sensor chip and/or the transparent substrate may be destroyed in some cases. Furthermore, the circuit conductor layer and the bumps (electrodes) may have poor electrical connection. The reasons are: the contact between the circuit conductor layer and the bumps may be insufficient because the contact is obtained through physical deformation of the bumps; it is difficult to substantially offset such factors as variations in the heights of the electrodes, bumps (electrodes), and the circuit conductor layer and warping of the image sensor chip and/or the transparent substrate.